H-like iron emission in narrow-line Seyfert 1 galaxies as a temperature diagnostic of accretion flows

We study the formation of the I-I-like iron (Fe XXVI) Ly alpha line at 6.97 keV in narrow-line Seyfert 1 galaxies in the framework of current models for accretion into a black hole. We find that Fe XXVI Ly alpha emission is most likely formed by resonant fluorescence resulting from the strong radiation field in the region near the black hole, or possibly in the interphase between a hot corona and the optically thick or disk. Alternatively, the line could be formed by collisional excitation at a temperature of approximately 12 keV in accretion flows dominated by advection or gravitational accretion flows with strong shocks. For either resonant fluorescence or collisional excitation, the equivalent width of the Fe XXVI Lya line is a powerful temperature diagnostic of the emitting region. We propose some observational tests to distinguish between collisional and resonant fluorescent excitation of the line.